Resolved verification boundary
Published 0fd884f59 on codex/runtime-port as Chris Breuer without coauthors; not merged to main. This is an evidence/documentation checkpoint with no runtime source change.
The intermittent first-upload sequence is a verified host-contention artifact, not a Home-specific HTTP/3 divergence. A serialized alternating run of the complete unchanged test/js/web/fetch/fetch-http3-adversarial.test.ts passed 12/12 Home processes and 12/12 pinned Bun processes. Every process reported 27/27 tests / 163 assertions under the original five-second case deadlines.
Three intentional eight-process waves then reproduced the historical failure shape in both binaries on the shared eleven-logical-CPU Darwin host: Home passed 11/12 processes and pinned Bun passed 5/12. Ordinary failures were 22 pass / 5 fail / 3 between-test errors, beginning with the first 64 KiB cases timing out at five seconds before later sizes recovered; one pinned Bun process reached 20 pass / 7 fail / 4 errors. An unrelated ReleaseFast Zig compile was simultaneously using one CPU and about 4.4 GiB, with other active system work. An lsof snapshot showed sixteen distinct UDP ports, excluding accidental socket/port sharing.
The verified correction is to serialize this heavyweight HTTP/3 corpus file and not overlap it with builds or other native corpus processes. No test deadline, workload, assertion, retry, sleep, protocol selection, skip, or fallback changed. The checked-in quic.c, quic.h, H3Client.zig, and all seven h3_client modules match pinned Bun source.
A UDP proxy that dropped the first client datagram also exercised retransmission in all four Home/Bun client-server directions. Thirty further Home/Home loss processes completed in about 0.25–0.28 seconds. One earlier cold diagnostic completed in 10.2 seconds; it is retained and not counted as a passing five-second corpus result.
Evidence: zig-out/h3-539-repro.json, h3-539-concurrent.json, h3-539-sockets.json, h3-539-sockets.lsof, h3-539-loss.json, per-process logs, and docs/docs/NATIVE_RUNTIME_BINDINGS.md. This closes the specific intermittent local verification observation. #530 remains open for Darwin integration, other platforms, and native dependency ownership; #66 remains open for whole-suite parity.
Historical observation
Earlier serial runs on the shared host alternated passes and first-upload failures in both published Home and pinned Bun. Passing retries did not erase those results. The controlled serialized/concurrent comparison above now explains and preserves that boundary without changing the upstream file.